Hollow-core fiber (HCF) testing explained: questions and misconceptions

Hollow-core fiber (HCF) is moving rapidly from research labs into real-world deployments—particularly for data center interconnects (DCI), high-performance computing, and latency-sensitive networks. While its physical structure differs fundamentally from conventional single-mode fiber, the objectives of fiber testing remain the same: 

• Ensure performance 
• Validate deployment 
• Support long-term reliability 

As interest in HCF grows, so do misconceptions around how it should—or shouldn’t—be tested. Below, we address the most common questions we hear from network operators, hyperscalers, and fiber manufacturers, based on real-world validation and field experience.

Is there a standard for hollow-core fiber (HCF) testing? 

At present, there is no dedicated industry standard written specifically for hollow-core fiber testing. However, reliable hollow-core fiber characterization can be achieved today by applying established fiber test principles through instrumentation and analysis tools adapted to its unique physical properties. 

In practice, HCF characterization follows the same fundamental testing principles used for conventional single-mode fiber (SMF), adapted to account for the unique physical properties of hollow-core designs —such as lower Rayleigh backscatter (RBS) and surface scattering as the dominant scattering mechanisms arising from nanoscale surface imperfections in the microstructured silica walls.  

Standards bodies are actively evaluating HCF, but until formal specifications are published, validated test methods and field-proven instrumentation are what matter most. 

This is where practical experience becomes critical. EXFO solutions have been validated on Band-Gap and various NANF-based hollow-core fiber in both laboratory and field trials, ensuring accurate and repeatable results under real-world conditions.

What is the goal of OTDR testing in the context of hollow-core fiber (HCF)? 

The objective of OTDR testing for hollow-core fiber remains consistent with traditional fiber: verify continuity, measure loss, locate events, and characterize splices. What changes is the level of performance required from the OTDR. 

HCF exhibits back scattering levels approximately 15 dB lower than standard SMF. As a result, OTDRs designed solely for conventional fiber may struggle to deliver usable traces—especially over long distances. EXFO’s hollow-core fiber–optimized OTDR test kit addresses this with exceptionally high dynamic range, enabling reliable characterization even on links exceeding 100 km.

Another key challenge unique to HCF is splice interpretation. Hollow-core splices introduce gas filling events (GFEs) that can extend for kilometers and distort traditional OTDR measurements. EXFO developed a patent-pending method that decouples the GFE signature from the fiber trace, allowing accurate splice loss and reflectivity measurements—something not achievable with conventional analysis alone. A full characterisation of these GFEs can provide valuable information about the quality of the installations, including not only splice losses but also the levels of hydrogen and carbon dioxide present in the fiber after splicing.

What critical parameter is often overlooked when validating hollow-core fiber (HCF)? 

Latency. When hollow-core fiber is introduced into a network, validating latency is just as important as characterizing dispersion—especially in hybrid architectures that combine SMF and HCF. In these scenarios, latency must be assessed as a function of wavelength to confirm that replacing portions of SMF with HCF delivers the expected end-to-end delay reduction across the full operating band. 

Transmission delay variation with wavelength is particularly important in latency-sensitive applications and cannot be inferred from CD measurements alone.

EXFO’s FTBx-570 CD/PMD analyzer is uniquely suited for this task, as it: 

• Supports refractive indices down to 1.00 
• Measures CD, PMD, and wavelength-dependent latency in a single-ended configuration 
• Achieves reaches exceeding 200 km on HCF, compared to SMF, thanks to HCF’s lower attenuation 

In practice, single-ended testing delivers significant operational advantages: no second unit, no second technician, and faster troubleshooting, particularly during early deployments and field validation scenarios.

Why should chromatic dispersion (CD) and polarization mode dispersion (PMD) be measured on hollow-core fiber (HCF)? 

A common misconception is that because HCF inherently exhibits low chromatic dispersion and minimal nonlinear effects, CD and PMD do not need to be verified. In practice, measurement and validation are still essential, especially in high-bandwidth applications. 

While CD in hollow-core fiber is typically below 4 ps/nm·km, PMD is generally higher than in conventional SMF fiber and can vary by manufacturer. In real-world deployments, networks often include hybrid links combining SMF and hollow-core fiber where cumulative dispersion effects must be fully understood.  

Verifying CD and PMD ensures predictable system performance, supports interoperability with existing infrastructure, and helps identify any deviations from expected behavior before service activation.

Can single-ended CD/PMD testing really work on hollow-core fiber? 

Yes—and it has been optimized, tested and validated in both lab and field trials. The FTBx-570 single-ended dispersion analyzer uses an end-of-fiber reflection, not RBS, to perform CD and PMD measurements. As a result, the low backscatter coefficient of hollow-core fiber does not limit measurement range. Only the reflectivity at the fiber end matters. 

In validation trials, EXFO successfully measured HCF links exceeding 100 km using a standard UPC connector—demonstrating that long-distance characterization can be achieved efficiently with a single setup. 

Dual-ended PMD measurements are not the norm for most HCF applications. Single-ended testing, by contrast, provides a highly practical, scalable, and field-friendly solution—especially when rapid validation or troubleshooting is required.

Can BER or latency testing be performed directly over hollow-core fiber? 

Absolutely. With the FTBx-88810 Series tester and the BERT application, EXFO enables end-to-end bit error rate (BER) and latency measurements from 1G to 800G directly over hollow-core fiber. BER validation is particularly important, as it allows network operators to detect transmission impairments or link degradation—including effects related to gas absorption in the hollow core—under real network conditions.  

Latency measurements complement BER testing by confirming that the expected delay reduction delivered by hollow-core fiber is realized at the service level, not just inferred from physical-layer characteristics. 

These measurements are essential for applications such as data center interconnects (DCI), financial trading networks, and other time-sensitive infrastructures—where error-free transmission is critical and microseconds matter.

What about attenuation profile measurements? 

Hollow-core fiber’s ultra-low attenuation is one of its most compelling advantages, but total insertion loss alone does not provide a complete picture of fiber performance. For HCF, attenuation can vary with wavelength due to fiber design, microstructured cladding geometry, and coupling conditions—particularly across the C and L bands used in high-capacity and DCI applications. 

Attenuation profile measurements make it possible to: 

• Verify that low-loss performance is maintained across the entire operating band 
• Identify wavelength-dependent loss features that may impact system margin 
• Compare measured performance against manufacturer specifications 
• Validate consistency between fiber spans, lots, or installation conditions 

This level of insight is especially important during technology trials, acceptance testing, and early deployments, where HCF behavior is still being closely evaluated. 

To support these use cases, EXFO offers complementary tools for attenuation profiling: 

• CTP10: provides high-resolution attenuation analysis for detailed fiber characterization in a lab environment 
• FTBx-5255 Optical Spectrum Analyzer: paired with a broadband source, enables  visualization of attenuation profiles across the C+L bands with 0.017 nm resolution 

Together, these solutions allow engineers to move beyond simple loss measurements and gain a wavelength-resolved view of attenuation behavior, supporting both R&D validation and field acceptance testing with greater confidence.

Can an OLTS be used to characterize hollow-core fiber? 

Yes—within the right context. OLTS testing is particularly useful for: 

• Continuity verification 
• Confirming correct fiber routing 
• Measuring total insertion loss and length on short links or cables without splices 

For field verification and installation checks, OLTS measurements using two MaxTester 945 units—configured as a light source and power meter—have been successfully used to confirm continuity, total loss, reflectivity, and length on hollow-core fiber.

Proven in the field, not just on paper

Hollow-core fiber represents a fundamental shift in fiber technology—but testing it does not require guesswork or unproven assumptions. EXFO’s HCF testing solutions have been validated through real-world trials in collaboration with fiber manufacturers and leading hyperscalers, ensuring accuracy, scalability, and operational relevance. 

By combining high-dynamic-range OTDRs, single-ended CD/PMD and delay characterization, flexible OLTS verification, and high-speed BER and latency testing, EXFO delivers a complete, field-ready approach to hollow-core fiber testing—without unnecessary complexity. 

As HCF moves from innovation to deployment, EXFO remains committed to supporting every phase of its lifecycle with solutions that are proven, practical, and trusted.